1. Field of the Invention
The present invention relates to a zoom lens ideal for video cameras and in particular, surveillance cameras.
2. Description of the Related Art
Conventionally, surveillance cameras, such as those for closed circuit television (CCTV) have been used to monitor unmanned facilities. Surveillance cameras capture images during the day using visible light and at night using near-infrared light. Therefore, a lens system that can be used day or night, i.e., a lens system that can handle both visible and near-infrared light is demanded for surveillance cameras.
Typically, in a lens system designed for the visible light range, chromatic aberration occurs in the near-infrared light range and images captured at night using near-infrared light are out of focus. Thus, a lens system that can correct chromatic aberration over a wide spectrum (from the visible light range to the near-infrared light range) such that the focal points of the spectrum become uniform, is preferable for use in a surveillance camera. A lens that is capable of magnification, is compact, and has a large focal ratio and high optical performance is yet more preferable.
Conventionally, zoom lenses have been proposed that are capable of handling light in the visible range to the near-infrared range and are mountable to a surveillance camera (see, for example, Japanese Patent Application Laid-Open Publication No. 2009-230122). The zoom lens disclosed in Japanese Patent Application Laid-Open Publication No. 2009-230122 includes sequentially from an object side, a first lens group having a negative refractive power, a diaphragm, and a second lens group having a positive refractive power. The first lens group includes sequentially from the object side, a negative meniscus lens, a biconcave lens, and a positive lens. Further, the second lens group includes 5 simple lenses.
Nevertheless, in recent years, in addition to being able to handle wavelengths over a wide spectrum, ranging from the visible light range to the near-infrared light range, a high zoom ratio is also demanded of lens systems for surveillance cameras. Further, high focal ratio lens systems capable of capturing sharp images even in dimly lit places have also come to be demanded. With rapid advances in increasing the pixels of imaging elements (CCD, CMOS, etc.), lens systems capable of capturing even finer details of an object, i.e., megapixel lens systems, have also come to be demanded. In particular, a megapixel lens system for a surveillance camera is demanded that over the entire zoom range, can favorably correct various types of aberration with respect to light in the visible range to the near-infrared range and that further has extremely high optical performance.
However, lens systems for surveillance cameras such as the conventional art disclosed in Japanese Patent Application Laid-Open Publication No. 2009-230122, have, at best, an unsatisfactory zoom ratio on the order of two-fold. Furthermore, the f value of a conventional lens system for a surveillance camera is, at best, on the order of 1.3, again unsatisfactory. Therefore, in attempting to realize a high zoom ratio and a high focal ratio, a significant problem arises in that achieving high optical performance on the megapixel level is difficult with conventional arts.
To solve the problems associated with the conventional arts above, an object of the present invention is to provide megapixel zoom lens that over the entire zoom range, can favorably correct various types of aberration occurring with light in the visible range to the near-infrared range, and that has a high zoom ratio and a high focal ratio.